A number of patent specifications, such as European Patent Nos. 00 00 086; 00 40 867, and 00 79 629; or Austrian Patent Nos. 372,099, 379,602, and 385,039 describe binder systems which are crosslinked by transesterification with active ester groups. The systems of the above-noted Austrian patents exhibit a high stoving reactivity when thus formulated and, on being stoved at 130.degree. C. to 140.degree. C., yield paint films of a high crosslinking density with a smooth surface and excellent corrosion resistance on zinc phosphatized sheet steel. The crosslinking density necessary to achieve this corrosion preventive effect, however, is recognized to have a negative influence on the mechanical properties and the adhesion to the substrate, particularly if there is no pre-treatment or only an inadequate pre-treatment of the substrate.
Numerous publications describe the crosslinking of cathodically depositable paint films by transurethanization via blocked isocyanate groups. By a suitable choice of blocking agent, the desired temperature range for crosslinking can also be achieved. Representative teachings are found in EP 02 06 071 and in Ep 01 18 743. With these systems, the deblocking of the isocyanate groups and, hence, the crosslinking occur suddenly at a specific temperature. As a result, the film has only limited flow properties. "Low-bake" combinations generally exhibit inadequate film surface qualities. Moreover, if the proportion of urethane structures is increased, a deterioration of the solubility characteristics of such systems occurs.
The use of blends of different crosslinking systems, as proposed in DE 34 32 233 A, is unsuccessful in providing both a low-temperature cure and good film properties in most instances since the incompatibility of the components results in separation occurring in the binder combination or in the paints produced from the binder combination. It is recognized that this disadvantage can be overcome by partial condensation of the transesterification component with the basic binder, but this reduces the deposition voltage and intensifies the susceptibility to hydrolysis of the other ester groups which are present.